1. Field of the Invention
This invention pertains to the general field of water purification. In particular, it provides a new method and apparatus for the continuous treatment and reuse of waste water.
2. Description of the Prior Art
In general, water for domestic as well as industrial use has been the subject of treatment and purification for a long time in order to remove toxic contaminants, bad odors and suspended solids. The water is typically treated by various means of filtration and chemical reaction designed to accomplish specific purification goals, as needed by particular users. The most common systems involve passing the water through layers of filtering media, to remove solid particles, and through chemically active media, to reduce the water's hardness, improve its pH, and extract undesirable contaminants.
Accordingly, water purification agents are generally categorized in terms of their function as filters, when designed to eliminate larger size suspended solids; clarifiers, when intended to remove smaller size particles and reduce opacity; softeners, to remove calcium and magnesium ions and reduce water hardness; and refiners, to extract chlorine, iron and organic compounds. Both anion and cation exchange resins are used to perform a multiplicity of these functions.
The separation of solutes from water by means of selectively permeable membranes is an effective and economical method of water purification. The processes of reverse osmosis, ultrafiltration, nanofiltration and electrodialysis all involve the use of membranes that have the ability to prevent the migration of most dissolved and suspended solids while allowing the passage of water through their pores.
This invention is directed to the use of novel combinations of these known methods of water purification in new applications for treating waste water and similar effluents. In particular, it discloses a new three stage process for reducing total dissolved solids, commonly referred to as TDS, and for disinfecting cooling tower water. This new process is intended to replace the current practice of chemical treatment and periodic disposal of cooling tower water to the sewer system.
During normal cooling tower operation, the water is continuously subject to evaporation by exposure to unsaturated air. During the contact between the two phases, heat is absorbed by evaporating water molecules and released from the liquid. The ensuing heat and mass transfer result in a cooler and more concentrated water phase in the cooling tower. Thus, as water evaporates, the tower water TDS increases from its original level in the influent municipal source and soon reaches concentrations that could result in damaging scaling in the equipment. The problem is further compounded by suspended particles trapped in the water from the air during the very active mass transfer occurring in the cooling process, all of which contributes to a continuous rise in TDS during normal operating conditions.
In order to prevent this scaling problem, antiscalent chemicals are added that allow a higher TDS level in the water without precipitation. Before this higher TDS level is reached, normally at a concentration twice the TDS level of the incoming water, a large portion of the tower water is dumped to the sewer and replenished with fresh water and new antiscalent. Typically as much as 30 percent of the daily water consumption, which also includes evaporation, is due to this periodic blow down. Where particularly hard water exists, this percentage can be as high as 50. The net effect of this operation is a corresponding reduction in TDS, but at the cost of wasted antiscalent additives and great consumption of water. In addition, other chemicals are routinely added to the system to disinfect and reduce algae growth. These chemicals must also be replaced after each water blow down, further adding to the costs of operating a cooling tower under the traditional water treatment methods.
It is the purpose of this invention to address all the problems mentioned above. It has been found that the processes of nanofiltration and reverse osmosis are capable of effectively reducing the total dissolved content in cooling tower water and of keeping it at levels safe for continuous operation. Both processes involve the use of membranes that have the ability to prevent the migration of most dissolved and suspended solids while allowing the passage of water through their pores. Accordingly, the method is used in combination with sand filtration and disinfection to treat cooling tower water in a close circuit, without any need for periodic disposal and replacement of large portions of the tower capacity. In addition, the new process functions efficiently without antiscalent additives. However, since most naturally occurring waters contain substances that can adversely affect the functioning of these membranes by reducing their permeability to water, thus decreasing the efficiency of separation and water throughput, pretreatment of the influent water is always desirable and often necessary.